1. Field of the Invention
This invention relates generally to drill bits used in drilling subterranean wells or in core drilling of such wells. The invention relates specifically to drill bits having a variable effective diameter which facilitates placement of the drill bit downhole and retrieval thereof. The drill bit of the present invention is particularly suitable for passing through narrow spots in the wellbore, through sluffing spots and through casing to drill an expanded wellbore therebelow. The invention may also be employed in drill bits having replaceable blades or other cutter support structures.
2. State of the Art
Equipment for drilling into the earth is well-known and long established in the art. The basic equipment used in drilling generally includes a drill bit attached to the bottom-most of a string of drill pipe and may include a motor above the drill bit for effecting rotary drilling in lieu of or in addition to a rotary table or top drive on the surface. In conventional drilling procedures, a pilot hole for the setting of surface casing is drilled to initiate the well. A smaller drill bit is thereafter placed at the bottom of the pilot hole surface casing and is rotated to drill the remainder of the wellbore downwardly into the earth.
Many types and sizes of drill bits have been developed especially to accommodate the various types of drilling which are done (e.g., well drilling and coring). A drill bit typically comprises a body having a threaded pin connector at one end for securement to a drill collar or other drill pipe, a shank located below the pin, and a crown. The crown generally comprises that part of the bit which is fitted with cutting means to cut and/or grind the earth. The crown typically has portions designated as the chamfer (the portion below the shank which flares outwardly from the shank), the gage (the annular portion of the cutting means below the chamfer which is usually concentric with the shank), the flank (a tapered portion of the cutting means below the gage), and the nose (the bottom-most portion of the cutting means and that which acts upon the bottom of the hole).
Drill bits include cutting elements for cutting the earth. The two major categories of drill bits are diamond drag bits, which have small natural diamonds or planar or polyhedral synthetic diamonds secured to certain surfaces of the bit body, and roller cone bits, which typically comprise at least two rotatable cones having carbide or other cutting elements disposed on the surfaces thereof. From time to time, the cutting elements of any drill bit become dull and must be replaced or the bit itself replaced. During drilling operations, drilling fluid or mud is pumped down into the hole to facilitate drilling and to carry away formation cuttings which have been cut away by the cutting elements.
From time to time during drilling of a well, the drilling activity will stop for a number of reasons. For example, another length or joint of drill pipe must periodically be added to the drill string in order to continue drilling. At other times drilling will stop because the drill bit may become lodged or jammed downhole, or the drill bit will have become dulled and will need to be replaced. In response to any of these scenarios, the drill bit must be brought out of the hole to either diagnose the reason for the stoppage or to replace the old, worn cutting elements with new elements.
It frequently occurs that when a drill string is tripped or brought out of a hole, the bit will become jammed downhole because of an encounter with debris or with an irregularity in the wall of the hole. Jamming is particularly prevalent when the wellbore includes a non-vertical segment, either inadvertently or by design, such as during highly-deviated or horizontal drilling. In the former case, during drilling, the bit may wander or move temporarily from a strictly-vertical orientation, resulting in a hole which curves away from the vertical. A phenomenon of this type, particularly where the departure from the vertical is abrupt, may be known as a "dogleg." In the latter instance, the wellbore is caused to depart from the vertical by use of a whipstock or by directional or navigational drilling bottom hole assemblies. In both cases, because of the curvature of the hole, tripping a state of the art drill bit in or out of the hole is often time-consuming or even impossible, in the latter instance necessitating the severance of the drill string at the stuck point, retrieval thereof, setting of a whipstock and drilling a new hole around the remaining portion of the drill string and the bit at the end thereof.
In some instances, due to drill bit cutter damage or unusual formation characteristics, boreholes may be drilled which are "under gage" (i.e., having an undersize diameter in comparison to the design diameter or gage diameter of the drill bit), or out of round as well as under gage. Subsequent removal of the drill string and, in particular, the bit in such situations is difficult to effect.
Thus, it would be an improvement in the art to provide a drill bit which includes cutting means which are variably positionable to expand to full or design gage while downhole and in an operative drilling mode and to retract when raised in the hole to facilitate tripping the drill bit in and out of the hole.
It would also be an improvement to provide a drill bit which will pass through a smaller diameter wellbore or casing and drill a larger, expanded diameter hole therebelow.
Expandable cutting means associated with drilling equipment have been known for many years, but such expandable cutting means have been directed to solving other problems encountered in drilling procedures. For example, expandable cutters attached to a drilling sub and located intermediate to the drill string have been used as apparatus to underream previously-drilled holes. Underreaming is a procedure well-known in the drilling industry to enlarge a portion of a previously-drilled hole below a point of restriction. Thus, underreaming apparatus are used to enlarge holes below a casing in order to place the next length of casing (see, e.g., U.S. Pat. No. 1,944,556 to Halliday, et al.; U.S. Pat. No. 2,809,016 to Kammerer; U.S. Pat. No. 4,589,504 to Simpson) or to enlarge a previously-drilled pilot hole in preparation for insertion of explosives therein (see, e.g., U.S. Pat. No. 4,354,559 to Johnson; U.S. Pat. No. 3,817,339 to Furse).
Drill bit assemblies directed to drilling a wellbore have been designed in which the cutting means grind out a diameter exceeding the diameter of the drill bit body or drill string. For example, in U.S. Pat. No. 1,468,509 to Overman, a wedge-shaped drill bit has corresponding slips which dovetail with the drill bit so that when the bit is lowered to the bottom, the slips slide upwardly to come into complementary registration with the body of the drill bit. Drill rollers designed to finely-crush or comminute the material in the bottom of the hole are positioned at a slight angle to a central longitudinal bore so that as the rollers turn, they drill out a diameter of earth slightly larger than the diameter of the drill bit. The rollers of Overman, however, do not expand outwardly from a vertical axis to achieve a diameter significantly in excess of that of the drill bit. Further, the elongated design of the Overman device would be disadvantageous in curved well conditions.
In U.S. Pat. No. 1,838,467 to Stokes, a drill bit assembly includes two cutter blades positioned within a bit head, both cutter blades moving from a retracted position within the bit head to an expanded position relative to the bit head when a spring biased plunger is forced downwardly to engage the cutter blades. Upward motion on the bit carrier housed within the bit head urges the plunger upwardly to move the cutter blades into a retracted position for tripping out of the hole.
Expandable cutter means in the prior art have not been specifically developed to facilitate easy removal of the drill bit from a hole, particularly under special drilling conditions such as non-vertical or curved holes. Therefore, it would be an improvement in the an to provide cutting means associated with a drill bit which are appropriately expandable and retractable under all drilling conditions and which do not require complex subassemblies within the bit head.